Field of the Invention
This invention relates to a hydrostatic axial piston machine with a cylinder barrel located so that it can rotate around an axis of rotation, in which cylinder barrel there is at least one piston bore in which a power unit piston supported on a cam disk is located so that the piston can be displaced longitudinally. The cylinder barrel is in contact with a control surface which is stationary relative to the housing and is formed on a control base. A kidney-shaped inlet connection and a kidney-shaped outlet connection are located in the control base. During rotation of the cylinder barrel around the axis of rotation, a displacement chamber formed between the piston bore and the power unit piston is alternately placed in communication with the inlet connection and the outlet connection. The axial piston machine can be operated as a pump and as a motor in the same direction of rotation of the cylinder barrel and the same direction of flow of the hydraulic fluid. When the axial piston machine is operated as a pump with a driven cylinder barrel it takes in hydraulic fluid via the inlet connection and delivers it into the outlet connection. When the axial piston machine is operated as a motor, it is driven by hydraulic fluid under pressure supplied via the input connection.
Description of Related Art
Axial piston machines which can be operated as a pump and as a motor in the same direction of rotation of the cylinder barrel and the same direction of flow of the hydraulic fluid are operated in an open loop and can be used in mobile machines. In operation as a pump, in which the axial piston machine takes in hydraulic fluid via the input connection from a reservoir and delivers it into a delivery line connected to the outlet connection, the axial piston machine is used to supply at least one consumer, such as the consumers of the working hydraulics of the machine. If the axial piston machine is pressurized at the inlet connection with hydraulic fluid under pressure, for example, from a hydraulic accumulator, the axial piston machine functions as a motor in the same direction of rotation of the cylinder barrel and in the same direction of flow of the hydraulic fluid, which makes it possible to start a drive motor, such as an internal combustion engine, which is in a drive connection with the axial piston machine as part of a start-stop function and/or to boost the drive motor by the output of an additional torque by functioning as a booster drive.
A drive system with an axial piston machine in a drive connection with an internal combustion engine when functioning as a pump can be used to supply the consumers of working hydraulics and when functioning as a motor can be used as a hydraulic starter to start the internal combustion engine and/or as a booster drive for the internal combustion engine is known from EP 2 308 795 A1. When operating as a pump, the axial piston machine is driven by the internal combustion engine and takes in hydraulic fluid via the inlet connection from a reservoir, which is delivered via an outlet connection to the consumers of the working hydraulics. The drive system also comprises a hydraulic accumulator which, when the axial piston machine is operated as a pump, can be filled and charged with hydraulic fluid by a charging circuit. When the axial piston machine is operated as a motor, it is driven by hydraulic fluid from the hydraulic accumulator, which can be placed in communication via a discharging circuit with the inlet connection, so that the axial piston machine functions in the same direction of rotation of the cylinder barrel and the same direction of flow of the hydraulic fluid as a motor which generates a torque on the crankshaft of the internal combustion engine.
Axial piston machines operated in an open loop are optimized for operation as a pump. To achieve the highest possible suction speeds of the axial piston machine when it is operated as a pump, the suction channel connected with the inlet connection at the control surface of the control base is provided with large flow cross-sections and the kidney-shaped inlet connection in the control base is provided with a large flow cross-section. The outlet connection in the control base and the delivery channel in communication with the outlet connection is provided with smaller flow cross-sections on account of the delivery pressures to limit the material stresses in the control base, which are a function of the delivery pressure. As a rule, the kidney-shaped outlet connection is also interrupted by a plurality of webs to reduce stresses and deformations on the control base in the vicinity of the outlet connection during operation as a pump.
If an axial piston machine optimized for operation as a pump, and in which the inlet connection in the control base is provided with a flow cross-section which is larger than the outlet connection in the control base, hydraulic fluid under pressure is delivered to the inlet connection for operation as a motor by placing the inlet connection in communication with a hydraulic accumulator, to transmit, in operation as a motor, a torque to a crankshaft of the internal combustion engine which is in a drive connection with the axial piston machine, a large pressure field is created in the vicinity of the inlet connection in a constructively required gap between the control surface provided with the kidney-shaped inlet connection and the end surface of the cylinder barrel which is in contact with the control surface of the control base. The pressure field, with a correspondingly high pressure at the inlet connection, causes a lifting up of the cylinder barrel from the control surface which cannot be prevented by a hold-down spring which is normally present in the axial piston machine and which presses the cylinder barrel against the control surface. To prevent a lifting up of the cylinder barrel from the control surface when the axial piston machine is operated as a motor, hydraulic fluid at only a limited pressure can be delivered to the inlet connection of the axial piston machine which is optimized for pump operation but is being operated as a motor, as a result of which the efficiency and the torque generated by the axial piston machine are severely restricted when it is operated as a motor.
In hydrostatic axial piston machines designed for operation as a pump, which are operated in an open loop, the terminal areas of the kidney-shaped inlet connection in the control base are semicircular and are rounded by corresponding small radii to achieve a large flow cross-section of the inlet connection. When the axial piston machine is operated as a motor, in which hydraulic fluid under pressure is delivered to the inlet connection, by placing the inlet connection in communication with a hydraulic accumulator, due to the pressure present in the inlet connection, forces are exerted on the cylindrical surfaces of the inlet connection in the control base that can exceed the allowable stresses in the material of the control base in the terminal areas of the kidney-shaped inlet connection and can lead to a destruction of the control base.
Therefore, it is an object of this invention to provide axial piston machines of the general type described above but which, when operated as a motor, can be operated at an elevated pressure at the inlet connection, and when operated as a motor can deliver a high torque.